1. Field of the Invention
The present invention relates to a crystal article, a method for producing the same, and a semiconductor device utilizing the same.
2. Related Background Art
Monocrystalline thin film employed in semiconductor electronic devices and optical devices has conventionally been formed by epitaxial growth on a monocrystalline substrate. It is already known that Si, Ge or GaAs can be epitaxially grown from liquid, gas or solid phase on a silicon monocrystalline substrate (silicon wafer), or a monocrystal of GaAs, GaAlAs etc. can be epitaxially grown on a monocrystalline GaAs substrate. The semiconductor thin film prepared in this manner is used for the manufacture of semiconductor devices, integrated circuits, semiconductor lasers and light-emitting devices such as LED's.
Ultra-high speed transistors utilizing two-dimensional electron gas and superlattice devices utilizing potential well, which are recently developed, have been enabled by high precision epitaxy such as molecular beam epitaxy (MBE) utilizing ultra high vacuum and metal organic chemical vapor deposition (MOCVD).
In the epitaxial growth on such monocrystalline substrate, the lattice constant and the thermal expansion coefficient have to be matched between the monocrystalline material constituting the substrate and the epitaxially grown layer. For example it is possible to epitaxially grow a monocrystalline silicon film on sapphire constituting an insulating monocrystalline substrate, but the defects in the crystal lattice at the interface due to the difference in lattice constant and the diffusion of aluminum, which is a component of sapphire, into the epitaxial layer have been drawbacks in the application for electronic devices and circuits.
The conventional method of monocrystalline thin film formation by epitaxial growth depends strongly on the substrate material. Mathews et al. investigated combinations of the substrate material and the epitaxially grown layer (J. W. Mathews, EPITAXIAL GROWTH, Academic Press. New York, 1975 ed.).
The size of the substrate is 6 inches at maximum in case of silicon wafers, and is generally smaller in case of GaAs or sapphire. Besides the cost per chip becomes higher as the monocrystalline substrate prepared by the crystal pulling method is associated with a high production cost.
Therefore, very limited kinds of substrate materials have been available in the conventional method in order to form a monocrystalline layer usable for preparing satisfactory devices.
On the other hand, three-dimensional integrated circuits, in which semiconductor devices are laminated in the perpendicular direction to the substrate for achieving higher degree of integration and higher level of functions are being actively developed. The development of large-area semiconductor devices, such as solar cells in which devices are arranged in an array on an inexpensive glass substrate and switching transistors for liquid crystal display pixels, is increasing year after year.
In these fields it is commonly required to form a semiconductor thin film on an amorphous insulator and to form thereon electronic devices such as transistors. Particularly a technology of forming monocrystalline semiconductors of high quality on an amorphous insulating material is needed.
When a thin film is deposited on an amorphous insulating substrate such as SiO.sub.2, the deposited film generally assumes an amorphous or polycrystalline structure, due to the lack of long-range regularity of the substrate material. An amorphous film has short-range regularity in the order of closest atoms, but lacks regularity in the longer range. A polycrystalline film is composed of monocrystalline grains without particular crystal direction, mutually separated at high grain boundaries.
As an example, the formation of a silicon film by CVD on SiO.sub.2 gives rise to an amorphous silicon film at a deposition temperature not exceeding 600.degree. C., or a polycrystalline silicon film with grain size ranging from several hundred to several thousand Angstroms at a higher deposition temperature. However the grain size of polycrystalline silicon and the distribution thereof vary significantly according to the forming process.
There has been obtained a polycrystalline film of large grain sizes in the order of a micron to a millimeter, by fusing and solidifying an amorphous or polycrystalline film with an energy beam such as of a laser or a rod heater (Single crystal silicon on non-single-crystal insulators, Journal of crystal growth Vol.63, No. 3, October 1983, edited by G. W. Gullen).
Also there have been obtained polycrystalline films of a large grain size in the order of a micron in solid phase, by abnormal grain growth or by secondary recrystallization with surface energy (T. Yonehara et al., Mat. Kes. Soc. Symp., P.517, Vol.25, 1984/Y. Wada et al., J. Electrochemi. Soc., Vo1.129, No.9, P.1999, 1979/L. Mei et al., J. Electrochemi. Soc. Vo1.129, No.8, P.1791, 1982/C. V. Thomson et al., Appl. Phys. Let. 44, No.6, P.603, 1984).
The mobility of electrons, measured in a transistor formed in the thin films of above-mentioned crystal structures, is about 0.1 cm.sup.2 /V.sec in amorphous silicon, 1-10 cm.sup.2 /V.sec in polycrystalline silicon with grain size of several hundred Angstroms, and is comparable to that in monocrystalline silicon, in case of polycrystalline silicon of large grain size obtained by fusion and solidification.
These results indicate that a device formed in the monocrystalline domain in a grain and a device formed across a grain boundary differ significantly in electrical performance. More specifically, a deposition film obtained by a conventional method on an amorphous substrate has an amorphous structure or a polycrystalline structure with grain size distribution, and the device formed thereon is significantly inferior in performance to that formed in a monocrystalline layer. Consequently such a device can only be used for a simple switch device, a solar cell, a photoelectric converting device or the like.
Also the method of forming a polycrystalline thin film of large grain size by fusion and solidification requires a long time to obtain large grain size, since the amorphous or monocrystalline thin film on each wafer has to be scanned with an energy beam, thus being unsuitable for mass production and for obtaining a large area.
FIGS. 17A to 17D illustrate process steps of forming a monocrystal, wherein a crystal 20A is grown by epitaxial growth on a small monocrystalline surface 12. Then, if the growth is continued under a depositing condition which does not form nuclei on the position surface 11, the crystal 20A continues to grow without unnecessary nucleus formation and grows also in the lateral direction as indicated by 20B, 20C, thus eventually covering the deposition surface 11, as deposition or crystal growth does not take place on said deposition surface 11.
Such crystal growing phenomenon has been reported in certain research reports, but the substrate is inevitably limited since an expensive monocrystalline substrate has to be used for obtain in the monocrystal 12.
As explained in the foregoing, the conventional crystal growing methods cannot easily produce crystals suitable for three-dimensional integration of devices or crystals of a large area, so that it has not been possible to easily and inexpensively produce monocrystal or polycrystal required for obtaining devices of satisfactory characteristics.
On the other hand, the semiconductor devices, represented by p-MOS transistor, are required to achieve higher performance, particularly stabler characteristics, for example stabler mobility of positive holes.